The present invention relates to doping IIB-VIA semiconductors during molecular beam epitaxy. In particular the invention relates to doping IIB-VIA semiconductors with group VA or oxygen free-radicals.
Molecular beam epitaxy is a deposition process in which atomic or molecular beams are used to deposit a film of material upon a substrate. In the past, doping IIB-VIA semiconductors with group VA elements using molecular beam epitaxy, or other deposition processes, has not been very successful. Typically, the resulting structure has a net acceptor concentration which is too low for most uses.
Light emitting diodes and semiconductor lasers are used in many electronic and optoelectronic systems such as communication, recording and display systems. Most of the current light emitting diodes and all the semiconductor lasers emit light in the infrared and red regions of the electromagnetic spectrum. It is desirable to have available shorter wavelength light emitting diodes and laser diodes. Blue and green light emitting diodes and lasers are necessary elements in full color displays; would permit increased recording densities in optical recording systems; would provide improved underwater communications; and could be used in plastic fiber based local area networks. Currently there exist no blue or green laser diodes and the available short wavelength light emitting diodes such as SiC and GaN are both costly and inefficient.
IIB-VIA semiconductors are well suited for the production of visible light emitters since their bandgap energies cover the visible spectrum and they have large radiative efficiencies. The fabrication of light emitting diodes and lasers requires the availability of both n-type and p-type material Unfortunately, it is very difficult to p-type dope the large bandgap IIB-VIA semiconductors. A notable exception is ZnTe which can only be doped p-type.
Some progress with p-type doping of the large bandgap IIB-VIA materials by molecular beam epitaxy has recently been reported (for example, see J. M. DePuydt, M. A. Haase, H. Cheng and J. E. Potts, Appl. Phys. Lett. 55 (11), Sep. 11, 1989, p. 1103-1105); K. Akimoto, T. Miyajima and Y. Mori, Jpn. Journ. Appl. Phys. 28 (4), Apr. 4, 1989, p. L531-534). The net acceptor densities achieved, however, are low and thus inadequate for the fabrication of efficient light emitting devices. Furthermore, the dopants are not desirable for other reasons. Desirable impurities for p-type doping of IIB-VIA's are the group VA elements (N, P, As and Sb). Past attempts at doping with the group VA elements, however, have shown that it is difficult to incorporate sufficient concentrations of these impurities by molecular beam epitaxy (see R. M. Park, H. A. Mar and N. M. Salansky, J. Appl. Phys. 58 (2), Jul. 15, 1985, p. 1047-1049) or that appreciable damage is inflicted to the crystal during growth (see T. Mitsuyu, K. Ohkawa and O. Yamazaki, Appl. Phys. Lett. 49 (20), Nov. 17, 1986, p. 1348-1350).
In terms of efforts to incorporate substitutional acceptor impurities in ZnSe epitaxial layers during crystal growth, the highest degree of reported success, until very recently, concerned Li-doping during molecular beam epitaxial growth (see M.A. Haase, H. Cheng, J. M. Depuydt, and J. E. Potts, J. Appl. Phys., 67, 448 (1990)). Two major problems, however, appear to hamper the employment of Li as a practical impurity in ZnSe. First, a net acceptor density of approximately 1.times.10.sup.17 cm.sup.-3 seems to represent the upper limit for Li-doping. At higher Li concentrations, strong compensation occurs which renders the ZnSe material highly resistive (see M. A. Haase, H. Cheng, J. M. Depuydt, and J. E. Potts, J. Appl. Phys., 67, 448 (1990)). Secondly, Li impurities are unstable in ZnSe at temperatures above approximately 275.degree. C. The latter problem manifests itself should device processing procedures necessitate heating the material beyond 275.degree. C. P-type behavior has also been reported employing the isoelectronic impurity, oxygen, as a dopant in ZnSe layers grown by molecular beam epitaxy (see K. Akimoto, T. Miyajima, and Y. Mori, Jpn. J. Appl. Phys., 28, L531 (1989)). However, net acceptor concentrations in ZnSe:O layers appear to be low, the largest net acceptor density reported so far being 1.2.times.10.sup. 16 cm.sup.-3 (see K. Akimoto, T. Miyajima, and Y. Mori, Jpn. J. Appl. Phys., 28, L531 (1989)). Nitrogen has also received attention as a candidate p-type dopant element in ZnSe. For example, Suemune et al (see I. Suemune, K. Yamada, H. Masato, T. Kanda, Y. Kan and M. Yamanishi, Jpn. J. Appl. Phys., 27, L2195 (1988)) reportedly measure hole concentrations around 7.times.10.sup.15 cm.sup.-3 in nitrogen-doped (using NH.sub.3) lattice-matched ZnS.sub.0.06 Se.sub.0.94 /l GaAs epitaxial layers grown by metal-organic vapor phase epitaxy. Prior art ZnSe layers are highly resistive since only small concentrations of uncompensated nitrogen impurities can be incorporated during crystal growth.